The present invention relates to a method and apparatus for transmitting failure information of a virtual path in an ATM network for all of repeaters and receivers connected downstream of signal transmission direction, which repeaters and receivers will be hereafter referred to in general as "downstream device".
In the transmission networks, when failure is detected at the multiplexed level, a signal, so called an AIS (alarm indication signal) is transmitted to downstream devices of multiplexed paths, so as to prevent failure from propagating. For example, as discussed in Teranishi and Kitamura "Design of Digital Network Transmission Installation", page 32, Electrical Communication Association, 1984, AIS is transmitted by making all multiplexed paths marked.
However, in ATM (Asynchronous Transfer Mode), since a virtual path which corresponds to the multiplexed path, is identified by the header of a cell, AIS has to be transferred in a form of the cell, as discussed in Ohta, Hadama and Satoh "Operation and Maintenance Information in ATM-based transport Networks", 1989 National Conference in Autumn of the Institute of Electronics, Information and Communication Engineers, B-261. The cell used for transferring the maintenance signals, such as AIS, is called as "OAM (operation and maintenance) cell".
When AIS is transferred with the OAM cell, the transfer frequency of OAM cells has to be set optimally. In general, the bandwidth which is assigned to the virtual path has a wide range such as from tens k b/s to tens M b/s. In particular, when the assigned bandwidth is narrow, there is a possibility that the bandwidth of the OAM cells could increase beyond the assigned bandwidth. When the AIS transferring OAM cells is transmitted with a velocity which is beyond the nominal capacity of the assigned virtual path, since the virtual path assigned for the AIS is multiplexed with other virtual paths in the downstream devices, the bandwidths of the other virtual paths are disturbed by the OAM cells. This possibly causes an increase in cell loss ratio.
For instance, as shown in FIG. 1, assuming that a virtual path A having an average capacity of 0.1 Mb/s and a virtual path B having an average capacity of 1.5 Mb/s are accommodated in input lines C and D, each having 1.6 Mb/s capacity, and that the virtual paths A and B are multiplexed in an output line of 1.6 Mb/s capacity. In an ATM multiplexer, a cell buffer is used to store the cells, when the sum of the instantaneous cell rates becomes higher than the output line capacity for a short time.
In FIG. 1, if the average rate of the OAM cells for the virtual path A from the input line C becomes higher than 0.1 Mb/s for a period longer than the short time mentioned above, the cells of the virtual path B will be discarded, because a total input cell rate higher than 1.6 Mb/s makes the cell buffer full. As a result, cell loss ratio at other virtual path becomes higher. On the other hand, when the transfer frequency of the AIS transferring OAM cell is set to be too low, delay in detection of AIS may be caused due to overlooking of the OAM cell in the downstream devices in the virtual path upon occurrence of failure.